From 10000-11000 feet, the hike climbs relatively steeply over well-behaved scree (perfect for fast descents). From 11000 to around 12800, the trail's incline lessens somewhat, and the dominant rock type becomes the blocky "a'a" lava rocks (more treacherous on descent).
The trail is well-marked with tall iron poles every 500 feet or so, making it nearly impossible to lose during the day.
A steep climb takes you from 12800 to 13100 feet, at which point you join a spur trail from the summit road. Soon after, the trail forks. The left fork leads to Lake Waiau (13020 feet). Take the right fork to reach the summit. The prominent cone to your east is not the summit proper, but rather 13441-foot Pu'u Hau Kea. Crest a 13200-foot saddle and slowly descend over 1/2 mile until you meet the summit road and obtain your first views of the telescopes.
It is customary to walk along the saddle road for the next mile, until 13700 feet. The true summit is separated from the rest of the summit plateau by approximately 1/4 mile and a small saddle. The final summit trail is located on the right side of the road, just before you reach the upper observatories. You may not be able to see it during heavy fog but a sign with a drawing of a hiker on it designates the start of the trail.
Bring enough water and snacks for 4-6 hours of hiking. Lake Waiau will provide a reliable water supply in a pinch and may not require treatment, though the algae that greens its waters may put off many prospective drinkers. There are public Porta Pottys at both the upper and lower observatories.